Switchable coherent pixel array for frequency modulated continuous wave light detection and ranging

ABSTRACT

A LIDAR transceiver includes a source input, coherent cells, and an optical switch. The optical switch is configured to switchably couple the source input to the coherent cells. At least one of the coherent cells includes an input port, an optical antenna, and a splitter. The input port is coupled to the optical switch and the splitter is coupled between the input port and the optical antenna. The splitter is configured to split a received portion of a laser signal into a local oscillator signal and a transmit signal, where the transmit signal is emitted through the optical antenna. A reflection of the transmit signal is received through the optical antenna as a reflected signal, where the splitter is further configured to output a return signal that is a portion of the reflected signal.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.17/486,692 filed Sep. 27, 2021, which is a continuation of InternationalApplication No. PCT/US2020/025042 filed Mar. 26, 2020, which claims thebenefit of and priority to six U.S. Provisional Applications includingU.S. Provisional Application No. 62/940,790 filed Nov. 26, 2019, U.S.Provisional Application No. 62/849,807 filed May 17, 2019, U.S.Provisional Application No. 62/845,149 filed May 8, 2019, U.S.Provisional Application No. 62/845,147 filed May 8, 2019, U.S.Provisional Application No. 62/826,536 filed Mar. 29, 2019, and U.S.Provisional Application No. 62/826,528 filed Mar. 29, 2019. The entiredisclosures of U.S. Non-Provisional application Ser. No. 17/486,692,International Application No. PCT/US2020/025042, and U.S. ProvisionalPatent applications 62/940,790, 62/849,807, 62/845,149, 62/845,147,62/826,536, 62/826,528 are hereby incorporated by reference as if fullyset forth herein.

TECHNICAL FIELD

This disclosure relates generally to frequency modulated continuous wave(FMCW) light detection and ranging (LiDAR), more particularly, to aswitchable coherent pixel array for FMCW LiDAR.

BACKGROUND INFORMATION

Conventional LiDAR systems use mechanical moving parts to steer thelaser beam. And for many applications (e.g., automotive) are too bulky,costly, and unreliable.

BRIEF SUMMARY OF THE INVENTION

A FMCW LiDAR transceiver is implemented on a photonic integratedcircuit. The FMCW LiDAR transceiver performs optical beam steering in atleast one dimension via a switchable coherent pixel array. In someembodiments, the FMCW LiDAR transceiver is part of a LiDAR chip thatincludes a plurality of FMCW LiDAR transceivers arranged in an array(e.g., linear array, two dimensional array, etc.). The FMCW LiDARtransceiver and/or the LiDAR chip may be part of a FMCW LiDAR system.The FMCW LiDAR system determines depth information (e.g., range toobjects within a field of view of the transceiver, velocity of theobjects, etc.) for the field of view of the transceiver.

In some embodiments, the FMCW LiDAR transceiver includes one or moresubarrays. A subarray may include an input port, an optical switch, aplurality of splitters, a plurality of mixers, and a plurality ofantennas. The input port is configured to receive a frequency modulatedlaser signal. The optical switch is configured to switchably couple theinput port to the optical antennas, thereby forming optical pathsbetween the input port and the optical antennas. For each optical pathfrom the input port to one of the optical antennas, a splitter of theplurality of splitters is coupled along the optical path. Each splitterconfigured to split a received portion of the laser signal into a localoscillator signal and a transmitted signal. The transmitted signal isemitted via the optical antenna and a reflection of the transmittedsignal is received via the optical antenna as a reflected signal. Thesplitter also outputs a return signal that is a portion of the reflectedsignal. For each splitter, a mixer of the plurality of mixers is coupledto receive the return signal and the local oscillator signal from thesplitter. The mixer is configured to mix the return signal and the localoscillator signal to generate one or more output signals used todetermine depth information for a field of view of the transceiver.

In some embodiments, a FMCW LiDAR system includes a LiDAR chip. TheLiDAR chip includes a FMCW LiDAR transceiver implemented on a photonicintegrated circuit. The photonic integrated circuit includes one or moresubarrays. A subarray may include an input port, an optical switch, aplurality of splitters, a plurality of mixers, and a plurality ofantennas. The input port is configured to receive a frequency modulatedlaser signal. The optical switch is configured to switchably couple theinput port to the optical antennas, thereby forming optical pathsbetween the input port and the optical antennas. For each optical pathfrom the input port to one of the optical antennas, a splitter of theplurality of splitters is coupled along the optical path. Each splitterconfigured to split a received portion of the laser signal into a localoscillator signal and a transmitted signal. The transmitted signal isemitted via the optical antenna and a reflection of the transmittedsignal is received via the optical antenna as a reflected signal. Thesplitter also outputs a return signal that is a portion of the reflectedsignal. For each splitter, a mixer of the plurality of mixers is coupledto receive the return signal and the local oscillator signal from thesplitter. The mixer is configured to mix the return signal and the localoscillator signal to generate one or more output signals used todetermine depth information for a field of view of the FMCW LiDARsystem. The FMCW LiDAR system also includes a lens positioned tocollimate the transmitted signals emitted via the plurality of antennas.The lens is also positioned to receive the reflected signals and couplethe reflected signals to the emitting optical antennas.

BRIEF DESCRIPTION OF THE DRAWINGS

Non-limiting and non-exhaustive embodiments of the invention aredescribed with reference to the following figures, wherein likereference numerals refer to like parts throughout the various viewsunless otherwise specified.

Embodiments of the disclosure have other advantages and features whichwill be more readily apparent from the following detailed descriptionand the appended claims, when taken in conjunction with the examples inthe accompanying drawings, in which:

FIG. 1 shows a schematic of a Switchable Coherent Pixel Array FMCW LiDARchip, according to one or more embodiments.

FIGS. 2 a-d shows four versions of coherent pixels, according to one ormore embodiments.

FIGS. 3 a-c shows a Switchable Coherent Pixel Array where an opticalcoherent detection block is shared between multiple coherent pixels,according to one or more embodiments.

FIGS. 4 a-c shows examples of an active optical switch of FIGS. 1 and 3a.

FIGS. 5 a-c illustrates how a Switchable Coherent Pixel Array steers anoptical beam for FMCW LiDAR operation, according to one or moreembodiments.

FIG. 6 shows a LiDAR chip with a plurality of parallel FMCW LiDARtransceivers arranged linearly, according to one or more embodiments.

FIGS. 7 a-c shows examples of mechanically assisted laser beam scanningin an Switchable Coherent Pixel Array based FMCW LiDAR system, accordingto one or more embodiments.

FIG. 8 shows a diagram of a first embodiment of a coherent pixel whichutilizes two polarizations of light to improve performance of a FMCWLiDAR system, according to one or more embodiments.

FIG. 9 shows a diagram of a second embodiment of a coherent pixel whichutilizes two polarizations of light to improve performance of a FMCWLiDAR system, according to one or more embodiments.

FIG. 10 shows how coherent pixels may be used in a focal plane array forFMCW applications, according to one or more embodiments.

FIGS. 11 a-d illustrates electrical wiring schemes for SwitchableCoherent Pixel Arrays, according to one or more embodiments.

FIG. 12 shows a system diagram of a Switchable Coherent PixelArray-based FMCW LiDAR system, according to one or more embodiments.

DETAILED DESCRIPTION

A FMCW LiDAR system determines depth information (e.g., distance,velocity, acceleration, for one or more objects) for a field of view ofthe system. The FMCW LiDAR system uses a switchable coherent pixel array(SCPA) on a LiDAR chip (e.g., a photonic integrated circuit). The LiDARchip may include one or more FMCW transceivers on the LiDAR chip (e.g.,each FMCW transceiver could be responsible for a different angular fieldof view within a field of view of the LiDAR System). The FMCW LiDARsystem splits a FMCW beam into a signal portion and a mixing portion.The signal portion is conditioned via a lens assembly and output into afield of view of the FMCW LiDAR system. The signal portion is reflectedoff of one or more objects in the field of view to form a reflectedsignal, and the reflections of the signal portion are detected by theFMCW LiDAR system. A portion of the reflected signal is mixed with themixing portion of the beam to directly measures range and velocity ofone or more objects within the field of view of the FMCW LiDAR system.

The FMCW LiDAR system transceiver is implemented on a photonicintegrated circuit. The photonic integrated circuit includes one or morebasic functional subarrays. Each subarray includes an input port, anoptical switch, a plurality of splitters, a plurality of mixers, and aplurality of antennas. The input port is configured to receive afrequency modulated laser signal. The frequency modulated laser signalmay be external to the transceiver, or in some cases is on the same chipas the photonic integrated circuit. The optical switch is configured toswitchably couple the input port to the optical antennas, therebyforming optical paths between the input port and the optical antennas.In some embodiments, the optical switch optically couples the frequencymodulated laser signal to each of the optical antennas one at time overa scanning period of the FMCW transceiver.

For each optical path from the input port to one of the opticalantennas, a splitter of the plurality of splitters is coupled along theoptical path. Each splitter configured to split a received portion ofthe laser signal into a local oscillator signal and a transmittedsignal. The transmitted signal is emitted via the optical antenna and areflection of the transmitted signal is received via the optical antennaas a reflected signal. The splitter also outputs a return signal that isa portion of the reflected signal. For each splitter, a mixer of theplurality of mixers is coupled to receive the return signal and thelocal oscillator signal from the splitter. The mixer is configured tomix the return signal and the local oscillator signal to generate one ormore output signals. A frequency of a beat tone resulting from themixing is proportional to a distance to a surface that reflected thelight from the LiDAR system. The one or more output signals are used todetermine depth information for the field of view of the LiDAR system.Depth information describes ranges to various surfaces within the fieldof view of the LiDAR system and may also include information describingvelocity of objects within the field of view of the LiDAR system.

Note that the LiDAR chip can steer the light emitted from the LiDARsystem in at least one dimension. And in some embodiments, the opticalantennas are arranged in two-dimensions such that the LiDAR chip cansteer the optical beam two-dimensions. Being able to steer the beamwithout moving parts may mitigate form factor, cost, and reliabilityissues found in many conventional mechanically driven LiDAR systems.

FIG. 1 shows a schematic of the Switchable Coherent Pixel Array (SCPA)FMCW LiDAR chip (11), according to one or more embodiments. The LiDARchip is a photonic integrated circuit. The chip can include a pluralityof basic functional subarrays (100). Each subarray (100) includes anoptical input/output (I/O) port (102) and an optional 1-to-K opticalsplitter (103), where K is an integer, and one or more SCPAs (101). The1-to-K optical splitter (103) may be passive or active. Each of theoptical I/Os is fed by a frequency-modulated light source provided by anoff-chip or on-chip laser. The optical power can be distributed on-chipthrough the optional 1-to-K optical splitter to reduce the number ofoptical I/Os. In the illustrated embodiment, the respective outputs ofthe 1-to-K optical splitter (103) feeds a corresponding SPCA 101. In theillustrated embodiments, each SCPA 101 includes M coherent pixels (105)and an optical switch network (104), where M is an integer. Note that insome instances one or more of the optical switch networks (104), theoptional 1-to-K optical splitter (103), or some combination thereof, maybe referred to simply as an optical switch. The optical switch isconfigured to switchably couple the input port 102 to the opticalantennas within the coherent pixels, thereby forming optical pathsbetween the input port and the optical antennas. The optical switch mayinclude a plurality of active optical splitters. In some embodiments,the optical switch optically couples the frequency modulated lasersignal to each of the optical antennas one at time over a scanningperiod of the FMCW transceiver.

The optical switch network (104) selects one or more of the M coherentpixels to send and receive the Frequency Modulated (FM) light forranging and detection. The coherent pixels can be physically arranged ineither one-dimensional (e.g., linear array) or two-dimensional arrays(e.g., rectangular, regular(e.g., non-random arrangement like a grid))on the chip. In some embodiments, the selected coherent pixel is able totransmit the light into free space, receive the returned opticalsignals, perform coherent detection and convert optical signals directlyinto electrical signals for digital signal processing. Note that thereceived optical signals do not propagate through the switch networkagain in order to be detected, and instead outputs are separately routed(not shown in the illustrated embodiment), which reduces the loss andtherefore improves the signal quality.

FIGS. 2 a-d shows four versions of coherent pixels, according to one ormore embodiments. The four versions of coherent pixels may be, e.g.,embodiments of the coherent pixels described above in FIG. 1 . In FIGS.2 a and 2 b , light from the optical switch network (e.g., the opticalswitch network 104) is provided to an optical input port (203) of thecoherent pixel. A bi-directional optical 2×2 splitter (202) splits thelight into 2 output ports, referred two as TX Signal (205) and LocalOscillator, LO (206). TX Signal (205) is sent out of the chip using anoptical antenna (200). The optical antenna is a device that emits lightfrom on-chip waveguides into free space or couples light from free spaceinto on-chip waveguides, such as a grating coupler, an edge coupler, anintegrated reflector or any spot-size converters. The optical antenna istypically polarization-sensitive with much higher emission/couplingefficiency for light with one particular polarization (e.g. TE). Theantenna is reciprocal and therefore it collects the reflected beam fromthe object under measurement and sends it back to the bi-directional 2×2splitter (202), which in turn splits it between ports 203 and 204. Thebi-directional optical 2×2 splitter (202) functions as a“pseudo-circulator” in this monostatic configuration where thetransmitter and receiver are collocated. The received signal out of port204 and LO 206 are mixed for coherent detection by an optical mixer,which can be a balanced 2×2 optical combiner (201) as in FIG. 2 a or anoptical hybrid (209) as in FIG. 2 b . Finally, a pair of Photo-Diodes(PDs) (207) in FIGS. 2 a and 4 PDs in FIG. 2 b convert the opticalsignals into electrical signals for beat tone detection. The version inFIG. 2 a is referred to as the Balanced Photo-Diode (BPD) version andthe one in FIG. 2 b as the hybrid version. The hybrid version providesin-phase and quadrature outputs (I/O), which can be used to resolvevelocity-distance ambiguities or enable advanced DSP algorithms in anFMCW LiDAR system. Using bi-directional optical 2×2 splitter as the“pseudo-circulator” may eliminate having a discrete circulator for everysingle pixel which is impractical for large-scale arrays with hundredsof pixels. Accordingly, the coherent pixels may reduce cost and formfactor significantly with a signal-to-noise ratio (SNR) penalty up to 6dB (as some of the guided optical power cannot be used for coherentdetection). For example, the received optical signal may be dividedbetween the port 203 and the port 204, of which the latter is used forcoherent detection. The coherent pixel designs, shown in FIG. 2 c andFIG. 2 d , address this limitation by introducing a polarizationsplitting antenna 210 into the new structure. Light from the opticalswitch network is provided to the optical input port (203) of thecoherent pixel. An optical splitter (212) splits the light into 2 outputports, referred two as TX Signal (215) and Local Oscillator, LO (214).TX Signal (215) is sent out of the chip directly using a polarizationsplitting optical antenna (210) with one polarization (e.g. TM). Theantenna collects the reflected beam from the object under measurement,couples the orthogonal polarization (e.g. TE) into the waveguide (213)and sends it directly to the optical mixer. In this case, the opticalsignal received by the antenna is not further divided by any additionalsplitters or the “pseudo-circulator.” The received signal out of port(213) and LO (214) are mixed for coherent detection by an optical mixer,which can be a balanced 2×2 optical combiner (201) as in FIG. 2 c or anoptical hybrid (209) as in FIG. 2 d . Finally, a pair of Photo-Diodes(PDs) (207) in FIGS. 2 c and 4 PDs in FIG. 2 d convert the opticalsignals into electrical signals for beat tone detection. This designrealizes a highly efficient integrated circulator for every singlecoherent pixel and enables on-chip monostatic FMCW LiDAR with ultrahighsensitivity. The details will be further discussed in FIGS. 8 to 10 . Insome embodiments, in the context of FIG. 1 , the coherent pixels ofFIGS. 2 a-d are such that each of the plurality of optical antennas hasa separate splitter, and each splitter is coupled along a respectiveoptical path between the optical switch and the corresponding antenna.

FIGS. 3 a-c shows a SCPA where an optical coherent detection block isshared between multiple coherent pixels, according to one or moreembodiments. As shown in FIG. 3 a , the chip (11) can include aplurality of basic functional subarrays (100). Each subarray (100)includes an optical I/O port (102) and an optional 1-to-K opticalsplitter (103), and one or more SCPAs (101). Each of the optical I/Os isfed by a frequency-modulated light source provided by an off-chip oron-chip laser. The optical power can be distributed on-chip through theoptional 1-to-K optical splitter (103) to reduce the number of opticalI/Os. Each of the 1-to-K optical splitters feeds an optional 1-to-Noptical switch network (107) that selects 1 out of N rows, where N is aninteger. Each row includes a coherent receiver block (306). An opticalswitch network (104) further selects one out of the M antennas (105),where M is an integer, to send and receive Frequency Modulated (FM)light for ranging and detection. The antennas can be physically arrangedin either one-dimensional (e.g., linear array) or two-dimensional arrayson the chip (e.g., rectangular array, regular array, etc.). In thisdesign, the selected antennas transmit the light into free space andreceive the returned optical signals passively. The coherent detectionfunction including optical mixing and optical-to-electrical conversionis done in the coherent receiver block (306).

Note that in some instances one or more of the optical switch networks(104), 1-to-N optical switch network (107), or some combination thereof,may be referred to simply as an optical switch. The optical switch isconfigured to switchably couple the input port 102 to the opticalantennas, thereby forming optical paths between the input port and theoptical antennas.

FIGS. 3 b and 3 c are examples of coherent receiver blocks (e.g., thecoherent receiver block (306)), which use the “pseudo-circulator” andbehave similar to the coherent pixel blocks in FIGS. 2 a and 2 c .Different from the scheme in FIG. 1 , the received optical signalspropagate through the 1-to-M switch network again in order to bedetected at the coherent receiver block 306. Compared with SCPA in FIG.1 , this design reduces the number of photodiodes considerably and hencereduces the number of electrical outputs and simplifies electricalrouting and/or packaging. Additionally, the pixel size shrinksconsiderably, allowing smaller pitch between pixels and enabling higherresolution for the FMCW LiDAR.

In some embodiments, in the context of FIG. 3 the coherent receiverblocks of FIGS. 3 b and 3 c are such that, for each optical switchnetwork (104) there is only one splitter (202) coupled between the inputport and the corresponding optical switch network (104).

FIGS. 4 a-c shows examples of the active optical switch (104) of FIG. 1and FIG. 3 a . A binary tree switch network and its individual switchcell (401) are depicted in FIG. 4 a . A 50/50 optical splitter (400)feeds two optical phase shifters (402) which tune a phase of each armusing control signals 403 and 404. The electrical control of the opticalswitch can be in a push-pull fashion using two controls or it can besingle-sided using only one control. The optical signals in the two armsare combined using an optical 2×2 combiner (405). Depending on thecontrol signals, constructive (deconstructive) interference occurs andhence the light is switched between the two outputs. The optical phaseshifters (402) can be but not limited to thermo-optic phase shifters orelectro-optic phase shifters. As depicted in FIG. 4 b , the switchnetwork can also be implemented with an array of Micro Ring Resonators,MRRs (410). The MRR only picks up optical signals from the main buswaveguide when the resonant frequency of the device is aligned with thelaser wavelength. Electrical control signals set the resonances of theMRRs in the array and hence select the output port through which the FMSignal is sent and received. Similarly, the switch network can also beimplemented with an array of Micro-ElectroMechanical System (MEMS)switches as in FIG. 4 c . The MEMS switch is configured to steer thelight from the main bus waveguide and therefore selects the output portthrough which the FM Signal is sent and received.

FIGS. 5 a-c illustrates how a SCPA steers an optical beam for FMCW LIDARoperation, according to one or more embodiments. In this example, asingle SCPA-based LIDAR transceiver (501) is used for illustration. TheLiDAR transceiver 501 includes a FMCW light source input (502), anoptical switch network (503), coherent pixel cells (504) and one or moreoptical antennas (505). The LiDAR transceiver (501) may be, e.g., theFMCW LiDAR chip (11) described above with reference to FIGS. 1 and 3 a.And a coherent pixel cell 504 may be, e.g., a coherent pixel 105 asdescribed above with regard to FIG. 1 . And in some embodiments, thecoherent pixel cell 504 may be composed from elements of FIG. 3 a (e.g.,the coherent receiver 304 one or more optical antennas and correspondingoptical paths therebetween).

In the illustrated embodiment, the optical antennas of the LiDARtransceiver 501 are placed at a focal distance of a lens system (507).The lens system (507) includes one or more optical elements (e.g.,positive lens, freeform lens, Fresnel lens, etc.) which map a physicallocation of each coherent pixel, to a unique direction. In someembodiments, the lens system (507) is positioned to collimate thetransmitted signals emitted via the plurality of antennas. The lenssystem (507) is configured to project a transmitted signal emitted froman antenna of the plurality of antennas into a corresponding portion ofthe field of view of the scanner module, and to provide a reflection ofthe transmitted signal to the antenna. Each optical antenna sends andreceives light from a different angle. Therefore by switching todifferent antennas, a discrete optical beam scanning is achieved asillustrated in FIG. 5 b and FIG. 5 c . For the FMCW LIDAR, a laser beam(508) scans across the targets (509) in the field-of-view, and thecoherent pixels in the LiDAR transceiver (501) generate electricalsignals which are then digitally processed to create LIDAR point clouds.In some embodiments, the lens system (507) produces collimatedtransmitted signals that scan the transceiver field of view along oneangular dimension (e.g., as shown in FIGS. 5 b and c ).

As shown in FIGS. 5 a-c , the coherent pixel cells 504 are arranged in alinear array. However, in other embodiments, the coherent pixel cells504 may have some other arrangement (e.g., two-dimensional, rectangular,etc.). Note—that in some embodiments a two dimensional arrangement maybe used to emit a plurality of transmitted signals from the plurality ofantennas, such that the plurality of transmitted signals scan in twodimensions a portion of a field of view of a scanner module (asdescribed below with regard to FIG. 12 ). For example, scanning in afirst dimension and a second dimension, and the scanner module field ofview is at 5 degrees or better along the first dimension and is 5degrees or better along the second dimension.

FIG. 6 shows a LIDAR chip (606) with a plurality of parallel FMCW LiDARtransceivers (501) arranged linearly, according to one or moreembodiments. As illustrated the LiDAR chip 606 includes 8 FMCW LiDARtransceivers (501) arranged in a linear array. However, in otherembodiments, the FMCW LiDAR transceivers (501) may have some otherarrangement (e.g., two-dimensional, rectangular, etc.). Each SCPA emitsand receives light (608) simultaneously and independently with theassistance of a lens system (607) over a corresponding angularfield-of-view (FoV) (depicted in the figure as the small double sidedarrow at the end of each dashed line). Each SCPA covers a certainangular FoV and provides a certain pixel rate for a FMCW LiDAR systemthat includes the LiDAR chip 606. Z parallel FMCW LiDAR transceivers(501) may cover Z times larger angular FoV and provide Z times fasterpixel rate, where Z is an integer. Wide FoV and fast pixel rates can beimportant for high-performance FMCW LiDAR systems.

FIGS. 7 a-c shows examples of mechanically assisted laser beam scanningin an SCPA-based FMCW LiDAR system, according to one or moreembodiments. In FIG. 7 a , a photonic chip (606) and a lens system (607)are both mounted on a rotating platform (701). The photonic chip 606 maybe an embodiment of the LiDAR chip 606, the LiDAR transceiver 501, orsome combination thereof. In the illustrated embodiment, the photonicchip (606) can achieve solid-state scanning in a first dimension (e.g.,vertically), and the rotating platform (701) can achieve 360 degrees inan orthogonal second dimension (e.g., horizontally). In FIG. 7 b , thephotonic chip (606) and lens system (607) is stationary and the laserbeams are steered by a moving mirror (702) (e.g. a galvo mirror). InFIG. 7 c , the photonic chip (606) and lens system (607) are stationaryand the laser beams are steered by a rotating a polygon mirror (703).The moving mirror (702) and/or the polygon mirror (703) may generally bereferred to as a scanning mirror. And the scanning mirror is configuredto scan the beams (transmitted signals) in a second dimension within afield of view of a scanner module (as described below with regard toFIG. 12 ), the second dimension orthogonal to the one angular dimension.

Although the photonic chip 606 can achieve all-solid-state beamsteering, and in some cases it could be in two-dimensions (e.g., opticalantennas arranged in 2-dimensional array), the overall field-of-view andaddressable positions of FMCW LiDAR can be greatly improved with theassistance of a mechanical device as illustrated in the examples.

FIG. 8 shows a diagram of a first embodiment of a coherent pixel (813)which utilizes two polarizations of light to improve performance of aFMCW LiDAR system, according to one or more embodiments. Input light(801) originating from a laser enters the coherent pixel and is split byan X/(1−X) splitter (802), also referred to as a splitter (802). X % ofthe light leaves the top port of the splitter, which constitutes the TXsignal, and (1−X) % of the light leaves the bottom port of the splitter,which constitutes the local oscillator (LO) signal. Depending on thesystem parameters, an optimal splitting ratio may be chosen. The TXsignal enters a polarization assembly 820. In the illustratedembodiment, the polarization assembly 820 includes a polarizationsplitter (803) and a polarization-insensitive free-space coupler (804).However, in other embodiments, e.g., as discussed below with regard toFIG. 9 , the polarization splitter (803) and a polarization-insensitivefree-space coupler (804) may be replaced with a singlepolarization-splitting vertical chip-to-free-space coupler. Thepolarization splitter (803), also referred to as a polarizer, whichseparates transverse electric (TE) and transverse magnetic (TM)polarized light. As an example, the input light in FIG. 1 may beTE-polarized. TM-polarized light can be used without modification ofthis idea. Because the TX signal light is TE polarized, the light iscoupled to a top port on the right-hand side of the polarizationsplitter (803). Light that is TM polarized leaves through a bottom porton the right-hand side of the polarization splitter (803). The TX signalleaving the polarization splitter (803) enters apolarization-insensitive free-space coupler (804) which generates afree-space beam of light (805) that has a linear polarization matchingthe TE field of the preceding optical circuit (813). Thepolarization-insensitive free-space coupler (804) is an example of anoptical antenna. For example, the polarization-insensitive free-spacecoupler could be a vertical grating, an edge coupler (e.g. inverselytapered waveguide) or an angled reflector. The free-space beam (805)propagates through a quarter-wave plate (806) which converts thelinearly polarized beam of light to a circularly polarized beam of light(807) The now-circularly-polarized light (807) propagates over adistance, which delays the light relative to the LO signal. This beamreflects off of a target surface (808), producing a reflected beam oflight (809). Depending on the surface properties, this reflected beammay maintain its circular polarization or its polarization may becomerandomized. The reflected beam of light (809) propagates back throughfree-space and a second time through the quarter waveplate (806). If thereflected beam (809) maintained its circular polarization, then thetransmitted beam (810) will have a TM polarization (with respect to theoriginating transmitting and receiving optical circuit (813)). If thereflected beam (809) has a randomized polarization, then the transmittedbeam (810) will have a random polarization. The transmitted beam (810)is coupled back into the coherent pixel (813) and propagates back intothe top right-hand port of the polarization splitter (803). If thereceived beam of light is TM polarized, all of the light will be coupledto the bottom-left port of the polarization splitter (803). If thereceived beam is randomly polarized, then nominally half of the opticalpower will be coupled to the bottom-left port. Light coupled to thebottom-left port of (803) enters the two-input-power optical mixer (811)which mixes the delayed received signal with the LO signal. The opticalmixer generates one or more electrical signals (812) which areinterpreted by the FMCW system. Removing the quarter-wave plate onlyaffects the system performance for polarization-maintaining targetsurfaces and does not affect the basic principle of this idea.

The polarization assembly (820) may be configured to, e.g., couple anoptical signal from a first waveguide (e.g., from (802)) to form thetransmitted signal; polarize the transmitted signal to have a firstpolarization; polarize the reflected signal (incoupled via (804)) basedon a second polarization that is orthogonal to the first polarization toform a return signal; and couple the return signal into a secondwaveguide (e.g., going toward (811)) for optical detection.

The coherent pixel (813) may be, e.g., the coherent pixel 105. Thecoherent pixel (813) may also be an embodiment of the coherent pixeldescribed above with reference to FIG. 2 a . Similarly, the coherentpixel (813) may also be an embodiment of the coherent pixel describedabove with reference to FIG. 2 b . For example, the bi-directionaloptical 2×2 splitter (202) may be replaced with the X/(1−X) splitter(802) and the polarization splitter (803), and the optical antenna 200would be replaced with the polarization-insensitive free-space coupler(804). And in the context of, e.g., a LIDAR transceiver, for eachX/(1−X) splitter, a polarizer splitter is coupled along the optical pathbetween the splitter and an optical antenna. And the polarizationsplitter is configured to: polarize the transmitted signal to have afirst polarization (e.g., TE); and polarize the reflected signal to formthe return signal such that the return signal has a second polarization(e.g., TM) that is orthogonal to the first polarization.

FIG. 9 shows a diagram of a second embodiment of a coherent pixel (912)which utilizes two polarizations of light to improve performance of aFMCW LiDAR system, according to one or more embodiments. The secondembodiment is substantially similar to the first embodiments, exceptthat the polarization splitter (803) and free-space coupler (804) withinthe polarization assembly 820 in FIG. 8 are replaced by a singlepolarization-splitting vertical chip-to-free-space coupler (903) asillustrated in FIG. 9 . This free-space coupler takes TE light from itsleft input and generates a free space beam (904) with TE polarization.TM light incident on the coupler, meanwhile, is coupled into the bottomport of the optical device, which is connected to the optical mixer(910). The functionality and/or structure of the rest of the system inthis second embodiment, labeled (901), (902), (904), (905), (906),(907), (908), (909), (910), and (911) is substantially the same as(801), (802), (805), (806), (807), (808), (809), (810), (811), and(812).

Note in FIG. 9 , the functionality of the polarization (820) and thepolarization-splitting vertical chip-to-free-space coupler (903) are thesame. The polarization assembly (820) may be configured to, e.g., couplean optical signal from a first waveguide (e.g., from (902)) to form thetransmitted signal; polarize the transmitted signal to have a firstpolarization; polarize the reflected signal (incoupled via (903)) basedon a second polarization that is orthogonal to the first polarization toform a return signal; and couple the return signal into a secondwaveguide (e.g., going toward (910)) for optical detection.

The coherent pixel (912) may be, e.g., the coherent pixel 105. Thecoherent pixel (912) may also be an embodiment of the coherent pixeldescribed above with reference to FIG. 2 c . Similarly, the coherentpixel (912) may also be an embodiment of the coherent pixel describedabove with reference to FIG. 2 d . For example, the optical splitter(212) may be replaced with the X/(1−X) splitter (902), and thepolarization splitting antenna (210) would be replaced with the singlepolarization-splitting vertical chip-to-free-space coupler (903).

FIG. 10 shows how coherent pixels may be used in a focal plane array(FPA) for FMCW applications, according to one or more embodiments. Acoherent pixel in FIG. 10 may be, e.g., the coherent pixel 813 and/orthe coherent pixel 912. The FPA employs coherent pixels to form a beamsteering apparatus. In FIG. 10 , light entering M input waveguides(1001) is split between N output waveguides (1003) by an M×N splitter(1002), where M and N are integers. The N output waveguides areconnected to an array of coherent pixels (1004). This array can be onedimensional or two dimensional depending on if one dimensional ortwo-dimensional beam steering is desired. Each coherent pixel (1005)emits TE-polarized light (1006) that propagates through a quarter-waveplate (1007) which converts the light to circular polarization (1008).The circularly polarized light passes through a lens (1009) which mayconsist of one or more lens elements. This lens converts thespatially-distributed circularly polarized beams of light to angledcircularly polarized beams of light (1010). The output angle of the lensdepends on the position of the input beam (e.g., determined in part on alocation of the coherent pixel (1005) that emitted the beam) and thelens (1009), enabling beam steering operation. The angled beams reflectoff of a target (1011). The diffuse reflected light returns towards thelens at the same angle (1012). This reflected light may retain itscircular polarization or become randomly polarized depending on theproperties of the target. The reflected beam of light passes backthrough the lens (1009) which maps the angle of the beam to a specificposition on the FPA. The transmitted beam (1013) passes back through thequarter-wave plate (1007). If the reflected light maintains its circularpolarization, then the transmitted light (1014) will be TM-polarized. Ifthe reflected light is randomly polarized, then the transmitted light(1014) will have a random polarization. The transmitted light (1014)passes is coupled back into the array of coherent pixels (1004), whichconverts the light into an electrical signal as described previously.

FIGS. 11 a-d illustrates electrical wiring schemes for SCPAs, accordingto one or more embodiments. The electrical wiring schemes may reduce anumber of electrical I/Os significantly for a photonic chip of a LiDARtransceiver. Scheme 1 is illustrated in FIG. 11 a and FIG. 11 b . Scheme2 is illustrated in FIG. 11 c and FIG. 11 d . In this example, a 1-to-83-stage binary tree switch network is shown where each switch has oneelectrical control signal and a coherent pixel array where each coherentpixel has two electrical outputs (e.g. I/Q signals). In Scheme 1,switches in a same stage are electrically connected together. With onlythree switch control signals, a LiDAR system can switch between any ofthe eight coherent pixels. All the I output signals from the coherentpixels are connected together as one shared output (RX_I) and all the Qoutput signals as another shared output (RX_Q). When only one coherentpixel is activated by the switch network, the remaining coherent pixelsreceive little light as their transmitter signals or their LO signals.Therefore, shared outputs represent correct signals from the activatedpixel with little crosstalk from adjacent pixels. In this example,Scheme 1 reduces the number of I/O signals to a minimum of five for atotal 7 switch inputs and 16 coherent pixel outputs. The reduction inelectrical I/Os becomes even more significant as the scale of the SCPAincreases and/or the number of parallel SCPA increases. In Scheme 2,more than one coherent pixel can be selected to transmit and receivelight simultaneously. In FIG. 11 c , the switch control signals andcoherent pixel output signals are split between the top and bottom halfthe 1-to-8 binary switch network, yielding 5 switch controls and 4receiver outputs. During operation, the first switch is controlled tohave 50/50 splitting ratio at the two outputs, delivering even opticalpower into the top and bottom half of the 1-to-8 switch tree. With theindependent control and readout capability for the top and bottom halfof the tree, one pixel from the top half and one pixel from the bottomhalf can be activated simultaneously. Scheme 2 can be adapted to Scheme1 by operating the first switch stage in the normal binary mode and itcan also arbitrarily control the splitting ratio of the first switchstage, providing a more flexible and potentially software defined beamscanning option at some hardware cost.

FIG. 12 shows a system diagram of a SCPA-based FMCW LiDAR system,according to one or more embodiments. A scanner module (1201) includesthe SCPA LiDAR chip (1205) with a single or a plurality of FMCWtransceiver channels and a lens system (1203) that includes one or moreoptical elements. In some embodiments, the lens system (1203) is anembodiment of the lens system (507).

The SCPA LiDAR chip (1205) includes one or more frequency modulatedcontinuous wave (FMCW) LiDAR transceivers that are implemented as one ormore photonic integrated circuits. A photonic integrated circuit for atransceiver may comprise an input port, a plurality of optical antennas,an optical switch, a plurality of splitters, and a plurality of mixers.

The input port is configured to receive a frequency modulated lasersignal. The optical switch is configured to switchably couple the inputport to the optical antennas, thereby forming optical paths between theinput port and the optical antennas. For each optical path from theinput port to one of the optical antennas, a splitter coupled along theoptical path and configured to: split a received portion of the lasersignal into a local oscillator signal and a transmitted signal, whereinthe transmitted signal is emitted via the optical antenna and areflection of the transmitted signal is received via the optical antennaas a reflected signal; and output a return signal that is a portion ofthe reflected signal. For each splitter, a mixer coupled to receive thereturn signal and the local oscillator signal from the splitter, themixer configured to mix the return signal and the local oscillatorsignal to generate one or more output signals used to determine depthinformation for a field of view of the LiDAR system (also referred to asthe field of view of the scanner module (1201).

In some embodiments, the lens system (1203) produces collimatedtransmitted signals that scan the scanner module (1201) field of viewalong one or more angular dimension (e.g., azimuth or elevation). Thescanner module (1201) has a field of view of 5 degrees or better alongthe one angular dimension. And in embodiments with a two dimensionalarrangement of the optical antennas (e.g., rectangular grid) signalsfrom the plurality of optical antennas may be scanned in two dimensionswithin the field of view of the scanner module (1201). For example,scanning in a first dimension and a second dimension, and the scannermodule (1201) field of view is at 5 degrees or better along the firstdimension and is 5 degrees or better along the second dimension. Notethat the two-dimensional scanning in the above example is done purely byselective use of different coherent pixels.

The scanner module (1201) may also include a scanner (1202) to assistlaser beam scanning and/or a quarter-wave plate (QWP) (1204) to improvepolarization-dependent sensitivity. The scanning mirror (1202) is ascanning mirror, e.g., as described above with regard to FIGS. 7 b and c. In embodiments that use the scanning mirror (1202), the scanner module(1201) field of view is at 5 degrees or better along the first dimension(scanned via selective use of coherent pixels) and is 10 degrees orbetter along the second dimension (scanned at least in part via movementof the scanning mirror (1202)). A light source for the LiDAR chip (1205)can be integrated directly onto the same chip or coupled through fibercomponents. As shown, the light source a FMCW laser source (1207) thatgenerates a frequency-modulated optical signal for FMCW LiDAR operation.The laser source (1207) can be further amplified by an optical amplifier(1206) to increase the range of the FMCW LiDAR. The optical amplifiercan be a semiconductor optical amplifier (SOA) chip or a Erbium-dopedfiber amplifier (EDFA). The FMCW laser source (1207) is controlled by alaser driver circuit (1208) which is typically a controllable low-noisecurrent source. Outputs of the coherent pixels go to an array oftransimpedance amplifier (TIA) circuits (1211). The on-chip switches arecontrolled by switch driver arrays (1210). The FMCW processing enginecan be implemented with one or a plurality of FPGA, ASIC or DSP chips,which contains the following functionalities: SCPA control andcalibration logic (1215), FMCW LiDAR frame management and point cloudprocessing (1214), multi-channel analog-to-digital convertors (1216),FMCW LiDAR DSP (1212), and FMCW laser chirp control and calibrationlogic (1213). In case of implementing the SCPA LiDAR chip (1205) with aCMOS silicon photonic platform, some or even all of the electricalcircuit functionalities can be implemented monolithically with thephotonic circuits on a single chip. The data output (1220) of the FMCWprocessing engine is depth information. Depth information may include,e.g., three dimensional position data of a typical LiDAR point cloud andother information that an FMCW LiDAR can measure such as velocity,reflectivity, etc.

As described above, wide FoV and fast pixel rates can be important forhigh-performance FMCW LiDAR systems. Note that the scanner module (1201)can target at least 100K points per second over the FoV of the scanningmodule (1201).

FIG. 12 shows an example LiDAR system. In alternative configurations,different and/or additional components may be included in the LiDARsystem. Additionally, functionality described in conjunction with one ormore of the components shown in FIG. 12 may be distributed among thecomponents in a different manner than described in conjunction with FIG.12 . For example, in some embodiments, the SCPA LiDAR chip 1205 may beseparate from the scanner module (1201).

Additional Configuration Information

The figures and the preceding description relate to preferredembodiments by way of illustration only. It should be noted that fromthe preceding discussion, alternative embodiments of the structures andmethods disclosed herein will be readily recognized as viablealternatives that may be employed without departing from the principlesof what is claimed.

Although the detailed description contains many specifics, these shouldnot be construed as limiting the scope of the invention but merely asillustrating different examples. It should be appreciated that the scopeof the disclosure includes other embodiments not discussed in detailabove. Various other modifications, changes and variations which will beapparent to those skilled in the art may be made in the arrangement,operation and details of the method and apparatus disclosed hereinwithout departing from the spirit and scope as defined in the appendedclaims. Therefore, the scope of the invention should be determined bythe appended claims and their legal equivalents.

Alternate embodiments are implemented in computer hardware, firmware,software, and/or combinations thereof. Implementations can beimplemented in a computer program product tangibly embodied in amachine-readable storage device for execution by a programmableprocessor; and method steps can be performed by a programmable processorexecuting a program of instructions to perform functions by operating oninput data and generating output. Embodiments can be implementedadvantageously in one or more computer programs that are executable on aprogrammable system including at least one programmable processorcoupled to receive data and instructions from, and to transmit data andinstructions to, a data storage system, at least one input device, andat least one output device. Each computer program can be implemented ina high-level procedural or object-oriented programming language, or inassembly or machine language if desired; and in any case, the languagecan be a compiled or interpreted language. Suitable processors include,by way of example, both general and special purpose microprocessors.Generally, a processor will receive instructions and data from aread-only memory and/or a random access memory. Generally, a computerwill include one or more mass storage devices for storing data files;such devices include magnetic disks, such as internal hard disks andremovable disks; magneto-optical disks; and optical disks. Storagedevices suitable for tangibly embodying computer program instructionsand data include all forms of non-volatile memory, including by way ofexample semiconductor memory devices, such as EPROM, EEPROM, and flashmemory devices; magnetic disks such as internal hard disks and removabledisks; magneto-optical disks; and CD-ROM disks. Any of the foregoing canbe supplemented by, or incorporated in, ASICs (application-specificintegrated circuits) and other forms of hardware.

What is claimed is:
 1. A LIDAR transceiver, comprising: a source inputconfigured to receive a laser signal; a plurality of coherent cells; andan optical switch configured to switchably couple the source input tothe plurality of coherent cells, wherein at least one coherent cell ofthe plurality of coherent cells includes: an input port coupled to theoptical switch; an optical antenna; and a splitter coupled between theinput port and the optical antenna, wherein the splitter is configuredto: split a received portion of the laser signal into a local oscillatorsignal and a transmit signal, wherein the transmit signal is emittedthrough the optical antenna and a reflection of the transmit signal isreceived through the optical antenna as a reflected signal; and output areturn signal that is a portion of the reflected signal.
 2. The LIDARtransceiver of claim 1, wherein the at least one coherent cell furtherincludes: a mixer coupled to receive the return signal and the localoscillator signal from the splitter, the mixer configured to mix thereturn signal and the local oscillator signal to generate one or moreoutput signals.
 3. The LIDAR transceiver of claim 2, further comprising:one or more processors configured to determine depth information for afield of view of the LIDAR transceiver based on the one or more outputsignals.
 4. The LIDAR transceiver of claim 2, wherein the one or moreoutput signals includes a quadrature output signal and an in-phaseoutput signal for the return signal.
 5. The LIDAR transceiver of claim2, wherein the at least one coherent cell further comprises at least onephotodiode coupled to the mixer to provide the one or more outputsignals as electrical signals.
 6. The LIDAR transceiver of claim 1,wherein a plurality of optical paths are respectively defined betweenthe source input of the transceiver and the plurality of coherent cells.7. The LIDAR transceiver of claim 1, wherein the at least one coherentcell further includes: a polarization assembly disposed between thesplitter and the optical antenna, the polarization assembly configuredto: couple an optical signal from a first waveguide to form the transmitsignal; and polarize the transmit signal to have a first polarization;polarize the reflected signal based on a second polarization to form thereturn signal; and couple the return signal into a second waveguide foroptical detection.
 8. The LIDAR transceiver of claim 7, wherein thefirst polarization is orthogonal to the second polarization.
 9. TheLIDAR transceiver of claim 1, wherein the plurality of coherent cellsare arranged in a linear array or a two-dimensional array.
 10. The LIDARtransceiver of claim 1, wherein the optical switch comprises: a passiveoptical splitter that splits the laser signal between at least twooptical paths.
 11. The LIDAR transceiver of claim 1, wherein the opticalswitch comprises: an active optical splitter that switchably couples thelaser signal to only one of at least two optical paths.
 12. The LIDARtransceiver of claim 1, wherein the optical switch optically couples thelaser signal to the plurality of coherent cells one coherent cell attime over a scanning period of the LIDAR transceiver.
 13. The LIDARtransceiver of claim 1, wherein the laser signal comprises a frequencymodulated continuous wave (FMCW) laser signal.
 14. An automotive LIDARsystem, comprising: at least one LIDAR transceiver that includes: asource input configured to receive a laser signal; a plurality ofcoherent cells; and an optical switch configured to switchably couplethe source input to the plurality of coherent cells, wherein at leastone coherent cell of the plurality of coherent cells includes: an inputport coupled to the optical switch; an optical antenna; and a splittercoupled between the input port and the optical antenna, wherein thesplitter is configured to: split a received portion of the laser signalinto a local oscillator signal and a transmit signal, wherein thetransmit signal is emitted through the optical antenna and a reflectionof the transmit signal is received through the optical antenna as areflected signal; and output a return signal that is a portion of thereflected signal; and a lens coupled to the LIDAR transceiver andconfigured to: collimate the transmit signal emitted by the opticalantenna; and receive and couple the reflected signal to the opticalantenna.
 15. The automotive LIDAR system of claim 14, wherein the atleast one coherent cell further includes: a mixer coupled to receive thereturn signal and the local oscillator signal from the splitter, themixer configured to mix the return signal and the local oscillatorsignal to generate one or more electrical output signals.
 16. Theautomotive LIDAR system of claim 15, further comprising: one or moreprocessors coupled to the LIDAR transceiver and configured to determinedepth information for a field of view of the LIDAR transceiver based onthe one or more electrical output signals.
 17. The automotive LIDARsystem of claim 15, wherein the one or more electrical output signalsincludes a quadrature output signal and an in-phase output signal.
 18. ALIDAR chip, comprising: a plurality of LIDAR transceivers, wherein atleast one LIDAR transceiver of the plurality of LIDAR transceiversincludes: a source input configured to receive a laser signal; at leastone coherent cell; and an optical switch configured to switchably couplethe source input to the at least one coherent cell, wherein the at leastone coherent cell includes: an input port coupled to the optical switch;an optical antenna; and a splitter coupled between the input port andthe optical antenna, wherein the splitter is configured to: split areceived portion of the laser signal into a local oscillator signal anda transmit signal, wherein the transmit signal is emitted through theoptical antenna and a reflection of the transmit signal is receivedthrough the optical antenna as a reflected signal; and output a returnsignal that is a portion of the reflected signal.
 19. The LIDAR chip ofclaim 18, wherein the at least one coherent cell further includes: amixer coupled to receive the return signal and the local oscillatorsignal from the splitter, the mixer configured to mix the return signaland the local oscillator signal to generate one or more electricaloutput signals.
 20. The LIDAR chip of claim 19, wherein the one or moreelectrical output signals includes a quadrature output signal and anin-phase output signal.